The present invention relates to the field of electronic sensing, i.e. electronic methods and apparatus for detecting force, displacement or other physical quantities. More specifically, the present invention relates to sensing methods and apparatus that automatically normalize acquired sensor data so as to compensate for variations in sensitivity from one sensor to another.
Force sensors, for example, can be used to provide feedback information in real-time machinery or process control applications. Another use of force sensing is in human interface applications such as isometric joysticks. Isometric joysticks are used for manual input of directional (and optionally other) information, for example, for cursor control on a computer display screen.
Isometric joysticks for cursor control can be arranged as a stand-alone or dedicated joystick positioned, for example, between the keys of a computer keyboard. Such arrangements are shown in T. Bentley and G. Meyer, Design and Evaluation of a Text Editing Console, SID 76 Digest, p. 67 (1976) and in PCT publication No. WO 92/09996. Alternatively, a joystick for cursor control can be implemented as part of a multi-purpose keyswitch, i.e. a keyswitch for both typing and pointing operations, as shown in U.S. Pat. No. 4,680,577 to Straayer et al.
Straayer et al. suggested use of pressure sensors or other force sensitive devices in a pointing device. One type of force sensor useful in pointing devices is a force-sensitive resistor ("FSR"). In a joystick that employs FSRs, forces applied by a user are detected as changes in resistance in the FSRs. Generally, three or four FSRs are symmetrically arranged around a shaft or pointing stick to resolve a direction of applied force in the X-Y plane. Downward force (Z direction) can also be detected in a suitable apparatus.
A commonly-assigned, co-pending application Ser. No. 07/672,641, filed Mar. 18, 1991 and entitled "Data Acquisition in a Multi-Function Keyboard System Which Corrects for Preloading of Force Sensors," discloses, inter alia, methods and apparatus for acquiring pointing data in a multi-function keyboard system. In one example of such a system, the keyboard includes a multi-purpose pointing key having a plurality of force sensors coupled to the key to acquire pointing information. Means are provided for selecting one at a time of a typing mode of operation for acquiring typing data, and a pointing mode of operation for acquiring pointing data. In the latter mode, the multi-purpose pointing key essentially acts as an isometric joystick for cursor control.
According to the prior invention, the force sensors are "preloaded" so that each of them exhibits an analog bias value, referred to herein as a "baseline response," when no external force is applied to the sensors by a user. The prior application teaches methods of determining the corresponding bias value for each of the force sensors, and deducting the force sensor bias values from the corresponding force sensor pointing values (readings while external force applied by a user) to determine net force values, thereby correcting for pre-loading of the force sensors.
However the force sensors might be arranged for a particular application, absolute amounts of force applied to a force sensor are difficult to measure due to variations in response characteristics or transfer curves from one force sensor to another. This is especially a problem in using force sensitive resistors (FSRs), for example, as the force to resistance characteristics of a given FSR model can vary as much as 60 or 70 percent from one sensor to another. Moreover, even if the absolute forces were unimportant, variations from one FSR to another within a single sensor assembly causes responses or gain that are direction dependent.
Since a direction indicated by a joystick user is determined essentially by summing forces applied to the individual sensors in the joystick assembly, each sensor having a corresponding direction, variations in response among the individual sensors leads to errors in resolving the direction and speed intended by the user--disconcerting problems to say the least. While the invention disclosed in the prior case discussed above is effective for correcting pointing data to remove the offset resulting from preloading, it does not teach how to compensate for the wide variation in sensitivities from one force sensor to another. As discussed above, these variations have an adverse effect on a pointing system.
It is possible to empirically characterize each and every force sensor to be used in a particular application. Then, suitable hardware and/or software could be arranged to normalize the individual responses from each such sensor in use. Such an approach would be neither practical nor economical and therefore is not commercially useful. Effective use of low-cost force sensors thus requires a simple and efficient way to compensate for these variations. The challenge, therefore, lies in collecting useful force data from a plurality of sensors, e.g. in a joystick, where each sensor has an individual response characteristic that is not well defined a priori, and the sensors exhibit a wide range of variability in response characteristics among them.